1. Field of the Invention
The present invention relates to a fuel cell stack comprising a cell assembly of unit cells each having a membrane electrode assembly which comprises an anode, a cathode, and a solid polymer ion exchange membrane sandwiched between the anode and the cathode.
2. Description of the Related Art
Usually, solid polymer electrolyte fuel cells employ an ion exchange membrane (solid polymer ion exchange membrane) comprising a polymer ion exchange membrane (proton exchange membrane). A membrane electrode assembly comprises an anode and a cathode, each made up of an electrode catalyst and a porous carbon panel, that are disposed on the opposite sides of the ion exchange membrane. The membrane electrode assembly is sandwiched between separators (bipolar plates), making up a unit cell for generating electricity. A predetermined number of such unit cells are stacked for use as a fuel cell stack.
When a fuel gas, e.g., a gas mainly containing hydrogen (hereinafter referred to as “hydrogen-containing gas”) is supplied to the anode, the hydrogen in the gas is ionized on the electrode catalyst and moves through the ion exchange membrane to the cathode. Electrons are supplied to an external circuit, which uses the electrons as an electric energy of a direct current. Since the cathode is supplied with a gas mainly containing oxygen (hereinafter referred to as “oxygen-containing gas”), for example, hydrogen ions, electrons, and oxygen react with each other on the cathode, producing water.
In the fuel cell stack, moisture contained in reactant gases, i.e., the fuel gas and the oxygen-containing gas, is occasionally condensed in each unit cell under some operating conditions including the flow rate, humidity, and temperature of those reactant gases. If the condensation occurs within reactant gas passages in the unit cells and within the membrane electrode assembly, then it tends to obstruct the supply of reactant gases and the discharge of the produced water, thereby increasing the concentration overvpotential and lowering the electric energy generating capability.
In particular, there has not been developed an effective technique of discharging condensed water from each of the unit cells of the cell assembly of a fuel cell stack. It has thus been desirous of increasing the ability to discharge the condensed water from those unit cells.